Budapest to Krakow overnight in own sleeper

Why don't you try calling the customer service number and discussing it with them.

The Man in Seat 61 has this suggestion and I confirmed that the Polrail site is offering double sleepers.

If booking online at mavcsoport doesn't work - it sometimes doesn't, either because the cheap fares are sold out or because MAV staff have forgotten to load the necessary data - either pick up the phone and book that way - although you'll only get full-flex tickets this way, not the cheapies as those are only sold direct by MAV or use reliable Polish agency www.polrail.com and pay for tickets to be sent to you. 

Here is an interesting take on night trains and an incredibly interesting blog on Eastern Europe: https://europebetweeneastandwest.wordpress.com/

A very interesting and enlightening article as it refers the future of services offered by MAV. If it should come to pass that MAV ends the sleeper car option on its night train between Munich and Budapest, a night route which I've yet to take in its entirety, does it automatically follow that MAV will abolish night service between Budapest and Munich? Hopefully not.

The article omits mentioning ÕBB's night route between Munich and Budapest on the EN train. It's a real pity that sleeper car service as an option will be abolished (for those wanting that, I don't), but as long as night service is in existence, ie sitting in the general seating area, I'm still in business in taking night train routes, such as those offered by EN.

If you are looking for old world elegance, not much left. There is the Danube Express, The South African Blue Train, the VSOE in Europe and Peru and maybe a hand full of others. Figure $1000 to $3000 pet journey day. The Hungarian rail system runs some special trains at reasonable rates, but mostly short haul of 4 hours or less. Then there are several short and medium haul special trains in the UK, but $$$$

Some of the trains out of Budapest are not modern and very slow. We didn't find their train service to be very pleasurable. I would tell you to fly there but only Wizz Air flies Budapest to Poland and then it's to Warsaw.

It's why I have yet to get yo Kracow. If I did want to go I think I would either put it first on the trip or last so I could use the international trip to take half the pain outnof it. Between Budapest and Kracow I would add a 2 to 3 day tour of Slovakia. There is another current post about how beautiful Slovakia is, and I agree with it.

You can use the portal about Polish trains
http://rozklad-pkp.pl/en